In the photographic industry, the need to provide photographic film and paper with antistatic protection has long been recognized. Such protection is important since the accumulation of static charges as a result of various factors in the manufacture, finishing, and use of photographic elements is a serious problem in the photographic art. Accumulation of static charges can result in fog patterns in photographic emulsions, various coating imperfections such as mottle patterns and repellency spots, dirt and dust attraction which may result in the formation of "pinholes" in processed films, and a variety of handling and conveyance problems.
To overcome the problem of accumulation of static charges it is conventional practice to provide an antistatic layer (i.e., an electrically-conductive layer) in photographic elements. A very wide variety of antistatic layers are known for use in photographic elements. For example, an antistatic layer comprising an alkali metal salt of a copolymer of styrene and styrylundecanoic acid is disclosed in U.S. Pat. No. 3,033,679. Photographic films having a metal halide, such as sodium chloride or potassium chloride, as the conducting material, in a hardened polyvinyl alcohol binder are described in U.S. Pat. No. 3,437,484. In U.S. Pat. No. 3,525,621, the antistatic layer is comprised of colloidal silica and an organic antistatic agent, such as an alkali metal salt of an alkylaryl polyether sulfonate, an alkali metal salt of an arylsulfonic acid, or an alkali metal salt of a polymeric carboxylic acid. An antistatic layer comprised of an anionic film forming polyelectrolyte, colloidal silica and a polyalkylene oxide is disclosed in U.S. Pat. No. 3,630,740. In U.S. Pat. No. 3,681,070, an antistatic layer is described in which the antistatic agent is a copolymer of styrene and styrene sulfonic acid. U.S. Pat. No. 4,542,095 describes antistatic compositions comprising a binder, a nonionic surface-active polymer having polymerized alkylene oxide monomers and an alkali metal salt. In U.S. Pat. No. 4,916,011, an antistatic layer comprising a styrene sulfonate-maleic acid copolymer, a latex binder, and an alkyl-substituted trifunctional aziridine crosslinking agent is disclosed. An antistatic layer comprising a vanadium pentoxide colloidal gel is described in U.S. Pat. No. 4,203,769. U.S. Pat. Nos. 4,237,194, 4,308,332, and 4,526,706 describe antistats based on polyaniline salt-containing layers. Crosslinked vinylbenzyl quaternary ammonium polymer antistatic layers are described in U.S. Pat. No. 4,070,189.
Frequently, the chemicals in a photographic processing solution are capable of reacting with or solubilizing the conductive compounds in an antistatic layer, thus causing a diminution or complete loss of the desired antistatic properties. To overcome this problem, antistatic layers are often overcoated with a protective layer to chemically isolate the antistatic layer and in the case of backside (that is, the side opposite to the photographic emulsion layer) antistatic layers, the protective layer may also serve to provide scratch and abrasion resistance for the photographic product and to prevent loss of antistatic properties due to a scratch disrupting the electrical continuity of the antistatic layer.
Typically, the protective layer is a glassy polymer with a glass transition temperature (Tg) of 70.degree. C. or higher that is applied from organic solvent-based coating solutions. For example, in the aforementioned U.S. Pat. No. 4,203,769 the vanadium pentoxide antistatic layer may be overcoated with a cellulosic protective layer applied from an organic solvent. U.S. Pat. Nos. 4,612,279 and 4,735,976 describe organic solvent-applied protective overcoats for antistatic layers comprising a blend of cellulose nitrate and a copolymer containing acrylic acid or methacrylic acid.
To apply the protective layer, the glassy polymers are normally dissolved in a solvent at very low solids to ensure low coating solution viscosities for good coatability at high coating speeds. Coating techniques employed include one to three layer extrusion dies (commonly referred to as X-hoppers), air knife, roller coating devices, meyer rods, knife over roll, and so on. For coating solutions comprising soluble polymers of reasonably high molecular weights, for example, larger than 50,000, the solution viscosity is a strong function of polymer concentration. For example, Elvacite 2041, a methyl methacrylate polymer sold by E. I. DuPont de Nemours and Co., has been described in the photographic art to form scratch protective layers for photographic materials. The polymer is normally dissolved in an organic solvent such as methylene chloride to form a clear solution. At concentrations above, for example, 4 to 5 weight %, the Elvacite 2041 solution viscosity is at least 20 centipoise at ambient temperature. Those viscosity values are too high for coating applications by, for example, certain roller coating or air-knife coating techniques, which require a coating solution viscosity in the range of from one to several centipoise. Therefore, photographic manufactures have to keep the solid concentration low to provide low solution viscosities and good coatability at high coating speeds.
Polymer solutions with low solids are useful for applications where lower dry coating coverages (less than about 300 mg/m.sup.2) can meet the physical and mechanical properties requirements for an imaging system. However, more advanced imaging applications need higher dry coating coverages for better physical and mechanical properties. To obtain high dry coating coverages, either more coating solution per unit area (wet coverage) has to be applied when using low viscosity/low solids polymer solutions, or higher viscosity/higher solids solutions must be used. As stated above, however, many coating applications cannot tolerate high viscosity/high solids polymer solutions, as such solutions cannot be coated at low wet coverages at high coating speeds. Some coating methods may allow one to coat high viscosity polymer solutions at high wet coverages, but they still suffer from several disadvantages. For example, in general, higher wet coverages mean more solvent recovery and higher cost for drying. Furthermore, due to both manufacturing limitations and various physical and mechanical property requirements for imaging element, wet coverages cannot be increased under certain conditions and for certain applications. For example, high wet coating coverages and the high levels of solvent retained in the film support as a result of these high wet coverages may have a significant impact on both dimensional stability and sensitometric properties of an imaging element. One may use resins of low molecular weight to lower the solution viscosity. However, the resultant dry coatings may not have adequate physical and mechanical properties.
Alternative approaches employing low viscosity, dispersed polymer particle-containing coating compositions have been described for paint and automotive coating industries. For example, U.S. Patent No 4,336,177 describes a solvent coating composition comprising non-aqueous dispersible composite polymer particles larger than 0.1 .mu.m. The particle has a core with a glass transition temperature (Tg) of about 10.degree. C. less than the polymerization reaction temperature. The particles are stabilized by block or grafting copolymers and can be transferred directly from aqueous medium to a non-aqueous medium. U.S. Pat. No 4,829,127 describes a coating composition comprising composite resin particles. Such particles are prepared by solution polymerization techniques in reaction vessels containing initiator, solvent, polymerizable monomers, and crosslinked particles. U.S. Pat. No 3,929,693 describes a coating composition comprising a solution polymer and polymer particles, where the polymer particles have a crosslinked rubbery core below 60.degree. C. and a grafted shell having molecular weight of 1,000 to 150,000. Reportedly, such coating compositions are more stable toward premature separation and flocculation. U.S Pat. No. 3,880,796 describes a coating composition comprising thermosetting polymer particles containing insoluble microgel particles having a particle size of from 1 to 10 .mu.m. U.S. Pat. No. 4,147,688 describes a dispersion polymerization process of making crosslinked acrylic polymer microparticles having a particle size of from 0.1 to 10 .mu.m. U.S. Pat. No. 4,025,474 describes a coating composition comprising a hydroxy-functional oil-modified or oil-free polyester resin, aminoplast resin, and 2 to 50% of crosslinked polymer microparticles (0.1 to 10 .mu.m) made by dispersion polymerization process. U.S. Pat. No. 4,115,472 describes a polyurethane coating composition comprising an ungelled hydroxy-containing urethane reaction product and insoluble crosslinked acrylic polymer microparticles (0.1 to 10 .mu.m) made by a dispersion polymerization process. Such coatings are reportedly useful for automotive industries.
There are significant differences in designing coating compositions for photographic applications from those for paint and automotive coating industries. The coating techniques and coating delivery systems are different so that they need different coating rheologies. The drying time in exterior and interior paint and architectural coating applications is on the order of hours and days, and in the automobile industry on the order of 10 to 30 min. However, in the photographic support manufacturing process the drying time for coatings is typically on the order of seconds. Often the drying time for solvent-borne coatings is as brief as 10-30 seconds for high speed coating applications. These differences put additional stringencies on the coating composition for photographic materials. For example, the coating viscosity frequently needs to be on the order of less than about 10 centipoise, and more often less that 5 centipoise, instead of on the order of one hundred to several thousand centipoise as in other coating industries. Film formation, dried film quality and transparency are especially critical. The tolerance on defects caused by polymer gel slugs, gelled particles, dust, and dirt is extremely low. This requires special precautions in delivery processes. The coating solutions need to be very stable toward, for example, high speed filtration and high shear.
U.S. Pat. Nos. 5,597,680, 5,597,681, and 5,695,919 describe coating compositions for imaging elements that contain core-shell polymer particles dispersed in liquid organic medium. Such coating compositions are stable and have low viscosity at high solids. However, it would be desirable to provide imaging elements with protective layers coated from organic solvent based coating compositions comprising other alternative polymers which yield dried layers having excellent physical and mechanical properties.
Aqueous coating compositions comprising water dispersible polymer particles have been reported to be useful for some applications. For example, they have been used as "priming" or subbing layers on film support to act as adhesion promotion layers for photographic emulsion layers, and used as barrier layers over, for example, a vanadium pentoxide antistatic subbing layer to prevent the loss of antistatic properties after film processing as described in U.S. Pat. No. 5,006,451. U.S. Pat. No. 5,679,505 describes an improved motion picture print film with a protective overcoat containing a polyurethane. Preferably the polyurethane is a water dispersible polyurethane. While these coating compositions are attractive from environmental considerations, the slow evaporation rate of water coupled with its extremely high heat of vaporization causes drying problems which are either not normally encountered or can be easily overcome in solvent-borne systems. Therefore, for manufacturing processes with conventional organic solvent drying capacity, the use of water-borne coating compositions often leads to very unsatisfactory results. In addition, solvent based coatings are preferred when the substrate or layer to be overcoated are moisture sensitive.
It can be seen that a coating composition useful as a protective overcoat for an antistatic layer must satisfy many unique requirements. The coating composition must allow the ability to apply thick dried layers from high solids, low viscosity formulations in order to protect the antistatic layer from diminution of its antistatic properties as a result of scratches and abrasions or exposure of the conductive materials to film processing chemicals. The coating composition must exhibit good potlife and be stable at high shear during filtration, delivery, and coating operations. The coating composition must also form high quality and highly transparent films under the extremely brief drying cycles used in photographic support manufacture. In addition, the coatings must be applied from environmentally acceptable solvents commonly used in the photographic industry. The present invention provides coating compositions which surprisingly meet all of these requirements while avoiding the problems and limitations of the prior art.